Optical transmission systems employ Wavelength Division Multiplexing (WDM) to increase information handling of an optical fiber transmission line, typically a long haul transmission line. In these systems, multiple signal bands are combined (multiplexed) onto a single transmission line. On reaching the receiving station, the signal bands are separated (demultiplexed) into separate channels.
Early WDM systems operated with a relatively narrow wavelength bandwidth, centered around 1550 nanometers, e.g. 1530-1565 nanometers, often referred to as the C-band. This is the wavelength region where standard silica based optical fibers have optimally low absorption.
In most WDM systems there is a trade-off between the number of channels the system accommodates and the channel separation. Both goals favor a wide operating spectrum, i.e. a wide range of operating wavelengths.
Recently, systems have been designed that extend the effective operating wavelength range well above the C-band transmission band. In terms of wavelength, the new band, referred to as the L-band, is variously defined, but for the purpose of this description is 1570-1610 nanometers. Use of these added wavelengths substantially extends the capacity of WDM systems. There is an ongoing effort to further extend the effective operating wavelength window to above 1610 nm, for example to 1620 nm. In WDM systems, it is important to have uniform gain over the entire WDM wavelength band. This objective becomes more difficult to reach as the operating wavelength range is extended to longer wavelengths.
It is often desirable to monitor the multiplexed signal in transit. Monitoring may provide several functions. Modern communication networks are designed for broad-band operation, but are still inherently wavelength sensitive. For example, signal loss is typically not uniform over a wide wavelength band. Monitoring the power in each band allows equalizer devices to operate. These may function by using the monitored power levels to provide feedback signals to channel amplifiers, typically at the WDM source.
While monitoring systems are important for system maintenance, signal taps of a multiplexed signal during transit may also involve using the information in the multiplexed signals. In a typical system maintenance application, the signals are analyzed for transmission performance but the information content is irrelevant. However, applications exist for tapping the signal content, for example in line drops, or for eavesdropping.
Conventional channel monitoring systems are usually variants of commercial spectrometers. Typical spectrometers rely on the interference of a finite number of beams that traverse different optical paths to form a signal. The spectrometer disperses the incoming light into a finite number of wavelength (energy) intervals, where the size of the resolution element is set by the bandwidth limit imposed by the dispersing element. Different dispersive techniques may be used. The two most common types of spectrometers are the grating spectrometer and the scanning Fabry-Perot spectrometer.
An example of a wavelength monitoring system using spectrometer principles applied to an optical fiber tap is described in U.S. Pat. No. 5,832,156 (incorporated herein by reference). Both Bragg grating and scanning Fabry-Perot spectrometer principles are implemented in that patent. The multiplexed signal, comprising all of the bands being monitored, is tapped from the optical fiber, and dispersion optics is used to spatially resolve the wavelength bands. The spatial resolution depends in each case on the amount of angular separation the optics can generate due to chromatic dispersion of the multiplexed beam.
Spectrometers in general are inherently complex and expensive. A large part of the expense is due to the optics used for resolving the different wavelength bands in the far field. With the trend in WDM systems toward more channels and less separation, the optics needed to resolve these wavelength bands becomes very demanding. A simpler and less expensive wavelength monitoring system would represent a significant contribution to the technology.